Sometimes when the moon orbits Earth, it moves between the sun and Earth. When this happens, the moon blocks the light of the sun from reaching Earth. This causes an eclipse of the sun, or solar eclipse. During a solar eclipse, the moon casts a shadow onto Earth.

There are three types of solar eclipses.

The first is a total solar eclipse. A total solar eclipse is only visible from a small area on Earth. The people who see the total eclipse are in the center of the moon’s shadow when it hits Earth. The sky becomes very dark, as if it were night. For a total eclipse to take place, the sun, moon and Earth must be in a direct line.

The second type of solar eclipse is a partial solar eclipse. This happens when the sun, moon and Earth are not exactly lined up. The sun appears to have a dark shadow on only a small part of its surface.

The third type is an annular (ANN you ler) solar eclipse. An annular eclipse happens when the moon is farthest from Earth. Because the moon is farther away from Earth, it seems smaller. It does not block the entire view of the sun. The moon in front of the sun looks like a dark disk on top of a larger sun-colored disk. This creates what looks like a ring around the moon.

During a solar eclipse, the moon casts two shadows on Earth. The first shadow is called the umbra (UM bruh). This shadow gets smaller as it reaches Earth. It is the dark center of the moon’s shadow. The second shadow is called the penumbra (pe NUM bruh). The penumbra gets larger as it reaches Earth. People standing in the penumbra will see a partial eclipse. People standing in the umbra will see a total eclipse.

Solar eclipses happen once every 18 months. Unlike lunar eclipses, solar eclipses only last for a few minutes.

The umbra (Latin for "shadow") is the innermost and darkest part of a shadow, where the light source is completely blocked by the occluding body. An observer in the umbra experiences a total eclipse.

The penumbra (from the Latin paene "almost, nearly") is the region in which only a portion of the light source is obscured by the occluding body. An observer in the penumbra experiences a partial eclipse.

“Total eclipses can only happen at the New Moon (every 29.53 days), during eclipse seasons (on average, every 173.3 days), and when the Moon is nearest to the Earth in its elliptical (non-circular) orbit (every 27.3 days). How often does it take for each of these cycles of different lengths to come around again like hands on a clock? Every 18 years, 11 days, and 8 hours the Moon will fully eclipse the Sun at the same node, at the same time of year, and with the Sun and Moon in nearly the same constellations as it did before.” Tyler Nordgren

Each Saros series has a number to identify it — for example, the August 21, 2017 eclipse belongs to Saros series 145. Eclipses within a Saros series are similar to each other, but different to eclipses of other series — they happen in different parts of the Earth, or are partial as opposed to total, etc. The Saros cycle itself isn't perfect; the various lunar cycles don't quite mesh up perfectly. For this reason, successive eclipses in a Saros series are shifted slightly either north or south (depending on the particular Saros) from each other. This means that a Saros series is actually of limited duration — about 70 to 85 eclipses over 1,200 to 1,500 years. Each series starts with a small partial eclipse in either the north or south polar regions; as the shadows of the successive eclipses move farther into the Earth, the first total eclipse will be seen near the polar regions. The eclipses of the series then march down or up the Earth, until the last total eclipse, and then a series of diminishing partial eclipses, occurs at the opposite pole to where the series started; and then it ends. (c) Moonblink.info

An exeligmos (Greek: turning of the wheel) is a period of 54 years, 33 days that can be used to predict successive eclipses with similar properties and location. For a solar eclipse, after every exeligmos a solar eclipse of similar characteristics will occur in a location close to the eclipse before it. (c) Wikipedia

The periodicity and recurrence of solar (and lunar) eclipses is governed by the Saros cycle, a period of approximately 6,585.3 days (18 years 11 days 8 hours). When two eclipses are separated by a period of one Saros, they share a very similar geometry. The two eclipses occur at the same node with the Moon at nearly the same distance from Earth and at the same time of year. Thus, the Saros is useful for organizing eclipses into families or series. Each series typically lasts 12 to 13 centuries and contains 70 or more eclipses. Every saros series begins with a number of partial eclipses near one of Earth's polar regions. The series will then produce several dozen central eclipses before ending with a group of partial eclipses near the opposite pole. The tables on the NASA website summarize the characteristics of solar eclipse Saros series 0 to 180 The number of eclipses in each series is given along with the series duration (years). Also listed are the calendar dates of the first and last eclipse in each series. Finally, the composition of each series is depicted by a sequence showing number and type solar eclipse. The Saros series number in the first column serves as a link to a catalog containing the circumstances for every solar eclipse in the Saros series as well as global maps for each eclipse. The eclipse sequence in the last column links to an animated GIF showing how the eclipse path changes with each member of the Saros.

An annular solar eclipse happens when the Moon covers the Sun's center, leaving the Sun's visible outer edges to form a “ring of fire” or annulus around the Moon. IT OCCURS WHEN THE MOON IS AT APOGEE (farthest from Earth).

A total solar eclipse happens when the Moon completely covers the Sun. Sometimes explosions of coronal gas on the sun are visible as well as Baily’s Beads--bits of light shining through valleys and mountains on the Moon.

A hybrid solar eclipse is a rare form of solar eclipse, which changes from an annular to a total solar eclipse, and vice versa, along its path.

BAILY'S BEADS

BAILY’S BEADS

The Baily's beads effect is a feature of total solar eclipses. As the moon "grazes" by the Sun during a solar eclipse, the rugged lunar limb topography allows beads of sunlight to shine through in some places, and not in others. The name is in honor of Francis Baily who provided an exact explanation of the phenomenon in 1836. The diamond ring effect is seen when only one bead is left; a shining diamond set in a bright ring around the lunar silhouette.

THE MEDES AND LYDIANS

LOUIS THE PIOUS

GEORGE DAVIDSON

SOLAR ECLIPSES IN HISTORY:

STONEHENGE: Built over a 1500-year period, archeologists have pushed back the date of the earliest construction to at least 3500 BCE, though a few have suggested a date much older--perhaps as far back as 5000 BCE. Regardless, its purpose appears to be communal, religious, and astronomical. Current research suggests that solar eclipses were predicted by the vast structure.

May 28, 585 BCE: Solar eclipse inspires truce between the Lydians and the Medes

According to the ancient Greek historian Herodotus, a total solar eclipse brought about an unexpected ceasefire between two warring nations, the Lydians and the Medes, who had been fighting for control of Anatolia for five years. During the Battle of Halys, also known as the Battle of the Eclipse, the sky suddenly turned dark as the sun disappeared behind the moon. Interpreting the inexplicable phenomenon as a sign that the gods wanted the conflict to end, the soldiers put down their weapons and negotiated a truce.

May 5, 840 CE: Solar eclipse scares Louis the Pious to death

The third son of Charlemagne, Louis the Pious inherited a vast empire when his father died in 814. His reign was marked by dynastic crises and fierce rivalry between his sons. A deeply religious man who earned his nickname by performing penance for his sins, Louis reportedly became terrified of an impending punishment from God after witnessing a solar eclipse. According to legend, he died of fright shortly thereafter, plunging his fractured kingdom into a civil war that ended with the historic Treaty of Verdun.

August 2, 1133 CE: Henry I of England

Perhaps one of the most famous total solar eclipses occurred on August 2, 1133 CE over England. While the totality of an eclipse normally lasts approximately two and a half minutes, this particular eclipse lasted for more than four minutes. The reason for this solar eclipse being so widely remembered is that it coincided with the death of King Henry I of England. Historian William of Malmesbury was quoted as saying that the “hideous darkness agitated the hearts of men.” William turned out to be right in a way. Shortly after King Henry’s death, the country was thrown into chaos and a civil war brewed. POSTSCRIPT: Malmesbury seemed to be getting his dates confused. The eclipse did occur in 1133 and Henry did voyage to France never to return to England, but most records show that he died in December of 1135. The important thing here is that after the eclipse the King disappears from English life, causing great insecurity among the people and hierarchy.

June 10, 1630 CE: Pope Urban and the Astrologers

Astrologers drew up charts and foretold Pope Urban VIII’s death—twice: A lunar eclipse in 1628 (Tommaso Campanella) and a solar eclipse in 1630 (Orazio Morandi). In both cases, Urban allowed magic to “forestall” the event. Yet in 1631, the Pope (out of fear and paranoia) forbade the use of astrology and magic, especially to foretell future events. His edict is still in effect.

TRIVIA: It is Pope Urban and the Roman Inquisition that put Galileo under house arrest in 1633 for promoting the idea of a heliocentric solar system, where he remained until his death in 1642. Urban survived until 1644, fourteen years after the supposedly fatal eclipse; Galileo was finally found “not guilty” of heresy in 1992 by Pope John Paul II.

August 18, 1868 CE: KING MONGKUT OF SIAM

Mongkut was King from 1851—1868; during his reign he strove to modernize/Westernize Siam (Thailand). He employed scientists and, most famously, teachers like Anna Leonowens. (We know much of this part of the story through the Rodgers and Hammerstein musical THE KING AND I) Mongkut died of malaria that he contracted while on an expedition with French scientists and the British Governor to the Malay Peninsula to witness the August 18, 1868 total eclipse. Unlike the Anna in the musical, the real Anna was away on October 1st, the day he died. Since 1982, August 18 has been celebrated as National Science Day in Thailand to commemorate both the accurate prediction of the eclipse and the King (The Father of Thai Science) who wanted to make his nation one of the focal points of scientific exploration.

George Davidson, a prominent astronomer and explorer, had already made surveys of several regions in Alaska–then a relatively uncharted territory–when he set out on a scientific expedition to Chilkat Valley in 1869. He was warned, however, that the local Chilkat Indians had been angered by some American provocation and might welcome him with guns and spears rather than open arms. During a tense initial meeting on August 6, Davidson explained that he had come for purely scientific reasons, telling the Chilkat that he was especially anxious to observe a total eclipse of the sun the following day. Right on cue, the sky grew dark over the Chilkat Valley as the moon eclipsed the sun. Apparently dismayed by this frightening display–some may have believed Davidson himself caused the eclipse–the Chilkat fled to the woods, leaving the scientists alone for the rest of their mission. Some historians believe the astronomer’s prediction may have saved the entire team from attack.

Clay tablets found at ancient archaeological sites show that the Babylonians not only recorded eclipses—the earliest known Babylonian record is of the eclipse that took place on May 3, 1375 BCE—but were also fairly accurate in predicting them. They were the first people to use the saros cycle to predict eclipses. The saros cycle relates to the lunar cycle and is about 6,585.3 days (18 years, 11 days, and 8 hours) long.

When distant planets in our galaxy or other galaxies pass in front of the star they’re orbiting, they cause dips in the amount of light we’re receiving. In essence, we’re viewing an eclipse. Not only can we determine the size of the planet, but spectral analysis can determine the planet’s atmosphere. Sensitive instruments on Hubble and other observatories can also see the heat signatures of these planets as they go “behind” the star during their orbits.

The further south from downtown, the closer you will be to totality. (For example: The South Water Front will get a fraction more of the eclipse than Montgomery Park.) In general, however, Downtown (just north and south of Burnside) will get 99.44% coverage--enough to significantly darken the sky. But that .56% will be enough to shed some light--similar to a deep twilight glow. The eclipse will begin at 9:06 AM, reach totality 10:19 AM (and last for around 2 minutes), and end at 11:38 AM.

Because of the geometry of the Earth’s shape, the Earth’s rotation speed, and the speed of the Moon as it orbits the Earth, the shadow will travel faster across the Earth’s surface at the beginning and the end of the eclipse path, and slowest right in the middle. By using one of the best eclipse calculators out there, we can see that the Moon’s shadow (also called the “Umbra”) is moving:

2955 mph in western Oregon

1747 mph in central Nebraska

1462 mph in western Kentucky

1502 mph near Charleston, SC

The eclipse ends far out in the Atlantic, where the shadow will once again be moving quickly.